The goal of this project are to study the biodistribution and in vivo pharmacokinetics of radiolabeled G-CSF in nonhuman primates. Preliminary work involved construction of 111-indium labeled G-CSF, followed by determination of the plasma clearance and half-life of the compound based on blood sampling. Biodistribution studies have been performed using gamma camera images and detected CPM to determine total body retention time, and organ localization as percent injected dose. Results from 1996 revealed good targeting to the hematopoietic space but greater than expected uptake in the kidneys. In 1997 we developed a number of G-CSF radioconjugates designed to decrease renal absorption. While we can effectively alter the biodistribution of G-CSF based on the method of radioconjugation and other biochemical modifications, we are still planning to test new modifications to find the optimal radioconjugate before moving into human trials.